Edwino: Well, it is all rather complex and I probably am not the person to do the topic justice. But I can throw out a few items:
- The Mahdi is far more central to Shi’a eschatology than it is the Sunni.
1a) The Imami Shi’a ( i.e. the current mainstream Iran, s.Iraq ), believe that the Mahdi will be Muhammed al-Muntazir ( called al-Mahdi ), who was supposedly occultated in 873.
1b) The Is’maili Shi’a believe that it will be Is’mail, who was supposedly occultated by God in 760.
1c) The Zaydi Shi’a ( N. Yemen, ~8 million followers) reject the concept of the Hidden Imamate ( occultations ) and the Mahdi altogether.
2)All information on the Mahdi are derived from the Hadith, not the Qur’an.
2a)The Shi’a and Sunni versions of the Haith differ, with the Shi’a version lending much stronger ( but obviously not unambiguous ) support for a Mahdi descended from the line of Muhammed via Fatima.
2b)The Sunni version is a matter of debate - Apparently two of the most respected compilers of the Sunni Hadith, Muhammed ban Ism’il Bukhari and Muslim ban Hajjaj Nishapuri, did not include the Mahdi tradition among their compilations. However several other Sunni scholars did, but they have been challenged as having weaker legitimacy. Nonetheless, the concept of the Mahdi seems to be widespread and at least somewhat accepted in the Sunni world. However…
Sunni eschatology differs on who the Mahdi will be. One line of thought is that he ( always male ) will be a normal man born shortly before the end times ( some similarity to the notion of the Jewish Messiah ). Another line of thought is that he has been living, unchanged, among the faithful for many centuries ( this may be my confusion with Shi’a tradition, however - I believe one tradition is that it was Muhammed ban Hanfiyya who has been living on Mt. Radwa since the 8th century ). Some think he will be synonymous with Jesus, others say they will be separate, with Jesus being a folower of the Mahdi. It all gets rather complicated.
There have been a number of ‘false Mahdis’. The most famous of course being Shaykh Muhammed Ahmad ( 1848-85 ), The Mahdi, who founded the Mahdist state of the Sudan.
4)Islam generally, Sunni and Shi’a, does have that rather Christian-like concept of an apocalyptic End Times. But what role, exactly, the Mahdi plays in them, other than an amorphous one where he will ‘fill the earth with justice and equity’, seems a bit unclear to me. Although I’m sure there are some bloodthirsty interpretations as to how he’ll do that, I believe that none of it is spelled out explicitly. That anti-Jewish screed is likely, as Collounsbury suggested, a combination of “imaginative” reading and wishful thinking.
So short answer? It’s kind of a mess. A mass of conflicting traditions, that aren’t even necessarily unchallenged even at their base ( some Sunni scholars think the whole notion didn’t surface until some decades after Muhammed’s death - Which would make them, de facto, irreligious ), though they do seem widely held in the Muslim world in one fashion or another.
Like I said, I’m probably not the best to tackle this one .